1. Technical Field
This invention relates to resistors which are monolithically integrated into a semiconductor substrate, and more particularly to an autobiased resistor, the input characteristics of which are independent of the voltage supplies of said substrate. It also relates to the use of such resistors in computer/peripheral equipment interfacing circuits.
2. Background Art
In the present state of the electronics art, there are many network configurations, wherein a computer, communications controller or other data processing unit is connected to up to several hundred peripheral units such as terminals, modems or printers. In such a network configuration, the interface circuit features must remain constant regardless of the number of peripheral units which are operating. Starting or stopping any peripheral unit must also not adversely affect operation of the other units. Moreover, the interface circuit must be capable of handling signal levels which are produced by different peripheral units manufactured by various manufacturers. Additional interface circuit operating parameters are determined from international standards (e.g., EIA RS-232-C) which define data exchange protocols, and set forth a set of hardware and software requirements.
Unfortunately, the data transmission field international standard EIA RS-232-C includes a number of requirements which make monolithic implementation of interface circuits difficult. For example, a particularly troublesome requirement is that the remote signals V.sub.in at the input of interface circuit receiver can vary from .+-.15 volts. However, the TTL logic circuits currently being used for interface circuits typically employ power supplies of +8.5 V and -8.5 V. Other logic circuit families may employ even lower power supply voltages. The requirement that the interface input circuitry accept input signals having higher values than the circuit supply voltage precludes monolithic integration of the interface circuits, as described below.
In planar monolithic integrated circuits, it is well-known that the maximum variations in the input and output signals must remain within values produced by the power supply circuits. Such a limitation arises from the fact that the devices (e.g., transistors, diodes or resistors) on the integrated circuit chip are isolated from one another by means of reverse biased PN junctions.
Thus, for example, an integrated circuit resistor is typically obtained by laying out a P-type diffused region, or body, in an N-type epitaxial layer. Clearly, in order for the PN junction thus formed to remain reverse biased, the potential in the epitaxial layer must be higher than any of the potentials existing along the resistor body, regardless of the signals applied to the resistor. Since a fixed voltage produced by the power supply circuits is typically applied to the epitaxial layer, all the signals at the resistor terminals must be of a lower voltage than the power supply voltage. This is clearly not possible under EIA RS-232C as described above, wherein a circuit having a power supply of .+-.8.5 V or less must accept input signals of up to .+-.15 V. Under such conditions, the PN junction isolation diode becomes forward biased and the resistor is no longer isolated so that the circuit will not function. Accordingly, such a requirement precludes monolithic fabrication of input resistors on the same substrate as the interface circuits.
A first solution to the problem of the supply voltages being lower than the input signal voltage is to provide an additional high voltage power supply circuit for the epitaxial layer isolation function. Such a solution is not desirable because of its high cost. An alternative solution is to employ discrete resistors for the interface circuit input resistors. Such a solution is costly, cumbersome and inherently less reliable than an all-monolithic approach.